This invention relates to an unbalanced hydraulic valve assembly (that is, it comprises a valve which is not pressure-equalized) including a first plunger element axially slidably held in an axial bore of a second plunger element and further, by means of a control port which is asymmetric relative to the circumference of the plunger elements, the flow of hydraulic liquid from a pressure source (generating the system pressure) to a work chamber is permitted or is blocked, dependent upon the relative position of the plunger elements. The first plunger element, because of its loose fit within the second plunger element, is radially displaceable to an extent which is, however, very small relative to its axial displacement.
A valve assembly of the above-outlined known type, disclosed, for example, in German Accepted Published Patent Application (Auslegeschrift) No. 2,602,050, may be of a hydraulic amplifier valve assembly and may find application particularly for a hydraulic brake system of an automotive vehicle.
The manufacture of conventional hydraulic valves, particularly hydraulic amplifier valve assemblies which include two plunger elements arranged for relative axial sliding motion within one another is relatively complex. By virtue of the symmetrical arrangement of the control ports relative to the circumference of the plunger elements, the conventional hydraulic valve is a balanced (that is, pressure-equalized) unit in which the plunger elements fit into one another with high precision in order to ensure, on the one hand, an easy axial displacement of one plunger element relative to the other and, on the other hand, to prevent leakage of the hydraulic fluid. For preventing leakages which, for example, cause a pressure drop in the hydraulic pressure accumulators cooperating with the valve assembly, it is known (as disclosed, for example, in the periodical Automobiltechnische Zeitschrift, 75 (1973) issue 7, on pages 261, 262 and in FIG. 2a) to provide in the path of the pressure supply conduit leading to the hydraulic valve, an additional pre-selection valve within the valve housing. During idle periods of the hydraulic valve, the pre-selection valve separates the hydraulic valve from the pressure source, but upon actuation of the hydraulic valve, the pre-selection valve establishes communication between the hydraulic valve and the pressure source by displacement of a pre-selection valve pin by cam action.
In order to reduce the construction costs, particularly with regard to the necessary precision of fit between the two relatively slidable plunger elements, without, however, adversely affecting their control or amplifying functions, the plunger elements which are arranged within one another for relative axial displacement, that is, the outer amplifier piston and the inner control plunger of a known hydraulic amplifier valve assembly are designed as an unbalanced unit, as disclosed, for example, in German Accepted Published Patent Application (Auslegeschrift) No. 2,602,050. The control plunger described therein is arranged within the amplifier piston with an intentionally loose fit so that the control plunger may execute radial motions which are, however, very small relative to its possible axial displacements. The control ports are asymmetrically arranged in relation to the circumference of the control plunger or the amplifier piston. The imbalance resulting from the radial displaceability and the asymmetrical arrangement of the control ports provides that during operation, dependent upon the position of the control plunger in relation to the amplifier piston, in the zone of the one or the other control port there are obtained, along the circumference of the control plunger, pressure differences which displace the control plunger in the zone of the respective control port radially against the inner wall of the amplifier piston bore. As a result, the control port, despite the loose fits, is closed with a highly effective seal.
In the above-outlined unbalanced hydraulic amplifier valve assembly annular seals are arranged on both sides of the pressure control port, at a certain axial distance therefrom. The effect of the annular seals is, among others, that at the height of the control port then closed, pressure may build up along the circumference of the control plunger in a manner discussed before; such a pressure causes a radial displacement of the control plunger.
Unbalanced hydraulic control plungers of the above-outlined type have been operating satisfactorily in practice and have the significant advantage that they may be manufactured with relatively loose fits. It is, however, a disadvantage of this known valve structure that the annular seals, because of the necessary frictions to assure a proper sealing effect, prevent an easy relative axial displacement of the plunger elements.